Project Summary/Abstract Protein methionine residues are emerging as essential redox sites for physiological processes spanning signal transduction, antioxidant defense, and regulation of protein function. At the same time, aberrant elevations in methionine oxidation induced by oxidative stress and/or inactivation of methionine sulfoxide reductase enzymes can contribute to neurodegenerative and vascular diseases, life span, and cancer progression. To help decipher contributions of methionine redox physiology and pathology, we are developing new chemical reagents to selectively label reactive methionine sites in proteins. The scientific premise is that providing access to a modular chemical toolbox for methionine bioconjugation will help unveil new functional methionine sites in proteins to further understanding of physiological and pathological contributions of methionine oxidation and reduction. Selective methionine bioconjugation using redox-based probes that emulate the innate oxidative chemistry of these residues offers chemical innovation, while demonstrating scientific rigor through the combined use of diverse and complementary synthetic, modeling, biochemical, proteomic, and imaging approaches to provide general tools for probing the reactive methionine landscape. Specifically, we seek to (1) design and synthesize robust and highly specific oxaziridine probes by establishing a predictive model for tuning selectivity and stability of methionine adducts, (2) identify oxidatively-sensitive methionine sites and biochemically characterize specific protein targets involved in reversible redox regulation by chemoproteomic comparison of cell models with or without the methionine sulfoxide reductase eraser protein on a proteome- wide scale, and (3) develop new imaging probes based on proximity ligation assays to monitor methionine- dependent redox status of selected protein targets in cells.